This proposal sets out to study the chemical reactions between compounds that are good inhibitor candidates and ?-lactamase enzymes. In the clinic, bacterial infections are becoming increasingly resistant to antibiotic-based treatments. A major source of resistance is the class of ? -lactamase enzymes produced by the bacteria. These enzymes hydrolyze, and thus inactivate, penicillin-like molecules before they can attack the bacteria. Here we set out to identify the intermediates formed by mechanism-based inhibitors that block the action of clinically problematic ? -lactamases. A focal point of the study will be the elucidation of the reactions of lead inhibitors with class A and class D ? -lactamases that function as carbapenemases (KPC-2 and OXA-24) and extended spectrum ? -lactamases (SHV-2, SHV-5 and OXA 10). A major thrust involves the use of Raman crystallography, where the reactions of the inhibitors within single crystals of the enzyme are followed in real time using a Raman microscope. This provides information on the identity and conformation of intermediates on the reaction pathway as well as kinetic data on their populations. The Raman data also provide an immediate link to X-ray crystallography with its detailed structural insights, and the two approaches are highly synergistic. In addition, the Raman analysis will be extended to solution studies, linking our knowledge of the reactions in crystals to the corresponding reaction in the aqueous phase. This is important because it is becoming clear that, for some inhibitors, the results from solution studies differs from those in crystals. Initially, our studies will use lead compounds that are 6-(pyridylmethylidene) penam sulfones possessing favorable IC50 values. Our results will identify reaction intermediates that block the active site under biologically relevant conditions and provide input into the design of better inhibitors, and thence potential therapeutic compounds.